With a permanent magnet electric motor in which a permanent magnet is built into a rotor, since the interlinkage magnetic flux of the permanent magnet is generated constantly at a given strength, the induced voltage generated by the permanent magnet will increase in proportion to the rotating speed. Thus, when performing variable speed operation from a low speed to a high speed, the induced voltage (counter electromotive voltage) generated by the permanent magnet will become extremely high in a high-speed rotation. When the induced voltage generated by the permanent magnet is applied to the electronic parts of an inverter and reaches their withstand voltage or higher, the electronic parts will break down. Thus, a design where the flux content of the permanent magnet is reduced so as to be the withstand voltage or less may be considered, but in this case, the output and efficiency of the permanent magnet electric motor will deteriorate in a low speed area.
Thus, proposed is technology of disposing, within the rotor, a permanent magnet of low coercive force of a level in which the magnetic flux density is irreversibly changed by the magnetic field created with a d-axis current of a stator winding (hereinafter referred to as the “variable magnetic force magnet”) and a permanent magnet of high coercive force having coercive force that is twice or more than that of the variable magnetic force magnet (hereinafter referred to as the “fixed magnetic force magnet”), and adjusting the total amount of interlinkage magnetic flux so that the total interlinkage magnetic flux generated by the variable magnetic force magnet and the fixed magnetic force magnet will decrease in a high revolution area where the power-supply voltage becomes a maximum voltage or greater.
Note that, since the flux content of the permanent magnet is decided based on the product of the coercive force and the thickness in the magnetization direction, when actually mounting the variable magnetic force magnet and the fixed magnetic force magnet in the rotor core, a permanent magnet in which the product of the coercive force and the thickness in the magnetization direction is small is used as the variable magnetic force magnet, and a permanent magnet in which the product of the coercive force and the thickness in the magnetization direction is large is used as the fixed magnetic force magnet. Moreover, generally speaking, an alnico magnet, a samarium-cobalt magnet (Sm—Co magnet) or a ferrite magnet is used as the variable magnetic force magnet, and a neodymium magnet (NdFeB magnet) is used as the fixed magnetic force magnet.
Incidentally, in this type of permanent magnet electric motor, when magnetizing a variable magnetic force magnet that was once demagnetized in a high revolution area, a phenomenon occurs where the magnetic field of the fixed magnetic force magnet disposed in the vicinity of the variable magnetic force magnet obstructs the magnetization magnetic field that is created by a d-axis current, and the d-axis current (magnetization current) for the magnetization increases by that much.